The present invention is directed to integrated circuits. More particularly, the invention provides a control system and method for signal sampling. Merely by way of example, the invention has been applied to real-time signal sampling in power conversion systems. But it would be recognized that the invention has a much broader range of applicability.
Generally, a conventional power conversion system often uses a transformer to isolate the input voltage on the primary side and the output voltage on the secondary side. To regulate the output voltage, certain components, such as TL431 and an opto-coupler, can be used to transmit a feedback signal from the secondary side to a controller chip on the primary side. Alternatively, the output voltage on the secondary side can be imaged to the primary side, so the output voltage is controlled by directly adjusting some parameters on the primary side. Then, some components, such as TL431 and an opto-coupler, can be omitted to reduce the system costs.
FIG. 1 is a simplified diagram showing a conventional flyback power conversion system with primary-side sensing and regulation. The power conversion system 100 includes a primary winding 110, a secondary winding 112, an auxiliary winding 114, a power switch 120, a current sensing resistor 130, an equivalent resistor 140 for an output cable, resistors 150 and 152, and a rectifying diode 160. For example, the power switch 120 is a bipolar junction transistor. In another example, the power switch 120 is a MOS transistor.
To regulate the output voltage within a predetermined range, information related to the output voltage and the output loading often needs to be extracted. For example, when the power conversion system 100 operates in a discontinuous conduction mode (DCM), such information can be extracted through the auxiliary winding 114. When the power switch 120 is turned on, the energy is stored in the secondary winding 112. Then, when the power switch 120 is turned off, the stored energy is released to the output terminal, and the voltage of the auxiliary winding 114 maps the output voltage on the secondary side as shown below.
                              V          FB                =                                                            R                2                                                              R                  1                                +                                  R                  2                                                      ×                          V              aux                                =                      k            ×            n            ×                          (                                                V                  o                                +                                  V                  F                                +                                                      I                    o                                    ×                                      R                    eq                                                              )                                                          (                  Equation          ⁢                                          ⁢          1                )            where VFB represents a voltage at a node 154, and Vaux represents the voltage of the auxiliary winding 114. R1 and R2 represent the resistance values of the resistors 150 and 152 respectively. Additionally, n represents a turns ratio between the auxiliary winding 114 and the secondary winding 112. Specifically, n is equal to the number of turns of the auxiliary winding 114 divided by the number of turns of the secondary winding 112. Vo and Io represent the output voltage and the output current respectively. Moreover, VF represents the forward voltage of the rectifying diode 160, and Req represents the resistance value of the equivalent resistor 140. Also, k represents a feedback coefficient as shown below:
                    k        =                              R            2                                              R              1                        +                          R              2                                                          (                  Equation          ⁢                                          ⁢          2                )            
FIG. 2 is a simplified diagram showing a conventional operation mechanism for the flyback power conversion system 100. As shown in FIG. 2, the controller chip of the conversion system 100 uses a sample-and-hold mechanism. When the demagnetization process on the secondary side is almost completed and the current Isec of the secondary winding 112 almost becomes zero, the voltage Vaux of the auxiliary winding 114 is sampled at, for example, point A of FIG. 2. The sampled voltage value is usually held until the next voltage sampling is performed. Through a negative feedback loop, the sampled voltage value can become equal to a reference voltage Vref. Therefore,VFB=Vref  (Equation 3)
Combining Equations 1 and 3, the following can be obtained:
                              V          o                =                                            V              ref                                      k              ×              n                                -                      V            F                    -                                    I              o                        ×                          R              eq                                                          (                  Equation          ⁢                                          ⁢          4                )            
Based on Equation 4, the output voltage decreases with the increasing output current.
FIG. 3 is a simplified diagram showing another conventional power conversion system with primary-side sensing and regulation. The power conversion system 200 includes a controller chip 202, a primary winding 210, a secondary winding 212, an auxiliary winding 214, a power switch 220, a current sensing resistor 230, an equivalent resistor 240 for an output cable, resistors 250 and 252, and a rectifying diode 260. The controller chip 202 includes a signal processing component 204, a demagnetization detector 206, an error amplifier 208, a reference-signal generator 248, an oscillator 228, a modulation component 218, a logic controller 224, an over-current-protection (OCP) component 226, and a driving component 222. The signal processing component 204 includes a sampling component 242, a switch 244, and a capacitor 246. The controller chip 202 includes terminals 282, 284, and 286. For example, the power switch 220 is a bipolar junction transistor. In another example, the power switch 220 is a MOS transistor.
The signal processing component 204 samples and holds a feedback signal 254 in response to a demagnetization-detection signal 256 from the demagnetization detector 206. The error amplifier 208 receives a sampled-and-held signal 258 from the signal processing component 204 and a reference signal 272 from the reference-signal generator 248, and outputs an amplified signal 262 to the modulation component 218. The modulation component 218 also receives a clock signal 264 from the oscillator 228 and a current-sensing signal 268 and outputs a modulation signal 266 to the logic controller 224. The driving component 222 outputs a drive signal 270 to the power switch 220 in order to regulate a primary current 272 flowing through the primary winding 210.
But errors can occur when the signal processing component 204 samples the feedback signal 254. Hence it is highly desirable to improve the techniques of primary-side sensing and regulation.